SUBTERRANEAN MOVEMENTS, &c. 119 
tion of the strata — which is the practical law of miners and colliers, and 
which I have verified by a multitude of examples in the superior orders 
of strata. The same reasoning would apply to the case of intersecting 
planes, one of which displaces the other, in a horizontal direction, but 
most of these are really explicable as cases of vertical movement of in- 
clined planes ; the apparent horizontal translation being proportioned in 
amount to the angle of inclination from the vertical of the plane dis- 
placed and the extent of its vertical displacement, and determined in 
direction by the direction of its dip. 
The variation of the hade of a plane of displacement, in strata 
of different nature, is to be explained by considering that in deter- 
mining the direction of the parts of this plane, the strata of least 
continuity or least resistance have the greatest influence, while those 
of most continuity must yield to the impulse of the others. While 
limestone completely jointed with wide joints yields in vertical planes, 
and oritstone partially so, laminated clays with close joints must often 
be found to yield obliquely. Such are the facts, and they are the 
more constantly observed in limestone tracts, because the laminated 
plates there occuring have oblique joints as well as vertical ones. 
The relation of throw of fissure and dip of strata is perhaps not 
a fact of sufficient generality to be made the basis of reasoning, yet 
if we consider that the angular elevation (producing dip) of the strata, 
the displacement of portions of them, and the corresponding hade of 
the vein, are all effects of the same action, we shall not be surprised to 
find these effects all happening in the same direction, which is the most 
general condition observed. 
Geological Periods. 
We come finally to examine a question, which has of late years 
risen to the highest importance, viz. the geological period of these 
disturbing movements. There is but one process for its solution : we 
must see what strata are, and what strata, under similar circumstances, 
are not dislocated. The former gives the minimum, the latter informs 
